PJM Interconnection LLC has declared a level one emergency for the 13-state eastern US grid and called on all power plants to operate at full capacity Thursday during a heat wave.

The largest US grid issued an Energy Emergency Alert level one, meaning it is concerned about being able to maintain adequate power reserves on July 27 as consumers and businesses turn up their air conditioners amid scorching heat. PJM has ordered all power plants to be online and for consumers enrolled in demand-response programs to be ready to curtail their electricity usage to keep power supply stable, according to a notice late Wednesday.

PJM, which serves more than 65 million people from Washington, DC to Illinois, warned of potential tight conditions earlier in the week and asked generators to prepare. Power suppliers that fail to show up when asked to by the grid may face stiff penalties.

Read More: America’s Biggest Power Source Wasn’t Built for Extreme Weather

(Corrects headline to clarify that emergency alert was declared.)

Author: Naureen S. Malik